ML4950
DESIGN CONSIDERATIONS
(Continued)
After the appropriate inductor value is chosen, it is
necessary to find the minimum inductor current rating
required. Peak inductor current is determined from the
following formula:
Capacitor Equivalent Series Resistance (ESR) and
Equivalent Series Inductance (ESL), also contribute to the
output ripple due to the inductor discharge current
waveform. Just after the NMOS transistor turns off, the
output current ramps quickly to match the peak inductor
current. This fast change in current through the output
capacitor’s ESL causes a high frequency (5ns) spike that
can be over 1V in magnitude. After the ESL spike settles,
the output voltage still has a ripple component equal to
the inductor discharge current times the ESR. This
component will have a sawtooth shape and a peak value
equal to the peak inductor current times the ESR. ESR also
has a negative effect on efficiency by contributing I
2
R
losses during the discharge cycle.
An output capacitor with a capacitance of 100µF, an ESR
of less than 0.1W, and an ESL of less than 5nH is a good
general purpose choice. Tantalum capacitors which meet
these requirements can be obtained from the following
suppliers:
AVX
Suitable inductors can be purchased from the following
suppliers:
Coilcraft
Coiltronics
Dale
Sumida
(847) 639-6400
(561) 241-7876
Sprague
(207) 282-5111
(207) 324-4140
I
L(PEAK)
=
t
ON( MAX)
V
IN( MAX)
L
MIN
(3)
In the single cell application previously described, a
maximum input voltage of 1.6V would give a peak current
of 383mA. When comparing various inductors, it is
important to keep in mind that suppliers use different
criteria to determine their ratings. Many use a conservative
current level, where inductance has dropped to 90% of its
normal level. In any case, it is a good idea to try inductors
of various current ratings with the ML4950 to determine
which inductor is the best choice. Check efficiency and
maximum output current, and if a current probe is
available, look at the inductor current to see if it looks like
the waveform shown in Figure 3. For additional
information, see Application Note 29.
If ESL spikes are causing output noise problems, an EMI
filter can be added in series with the output.
INPUT CAPACITOR
(605) 665-9301
(847) 956-0666
Unless the input source is a very low impedance battery, it
will be necessary to decouple the input with a capacitor
with a value of between 47µF and 100µF. This prevents
input ripple from affecting the ML4950 control circuitry,
and it also improves efficiency by reducing I
2
R losses
during the charge and discharge cycles of the inductor.
Again, a low ESR capacitor (such as tantalum) is
recommended.
XFMRS, Inc. (317) 834-1066
OUTPUT CAPACITOR
The choice of output capacitor is also important, as it
controls the output ripple and optimizes the efficiency of
the circuit. Output ripple is influenced by three
parameters: capacitance, ESR, and ESL. The contribution
due to capacitance can be determined by looking at the
change in capacitor voltage required to store the energy
delivered by the inductor in a single charge-discharge
cycle, as determined by the following formula:
SETTING THE OUTPUT VOLTAGE
The adjustable output can be set to any voltage between
2V and 3V by connecting a resistor divider to the SENSE
pin as shown in the block diagram. The resistor values R
1
and R
2
can be calculated using the following equation:
V
OUT
=
0.2
D
V
OUT
t
ON2
V
IN2
=
2
L
C
V
OUT
-
V
IN
1
6
(4)
1
R
+
R
6
1
2
R
2
(5)
For a 1.2V input, a 2.5V output, a 27µH inductor, and a
47µF capacitor, the expected output ripple due to
capacitor value is 11mV.
The value of R
2
should be 40kW or less to minimize bias
current errors. R
1
is then found by rearranging the
equation:
R
1
=
R
2
V
-
1
0.2
OUT
(6)
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
